Fuel injection systems based on common rail technology provide important advantages to engine and vehicle manufacturers who are under continual pressure by environmental regulatory bodies to reduce the pollution caused by the engine whilst improving the performance of the vehicle offered to the end user.
Principally, common rail technology enables the amount of fuel delivered to the combustion cylinders of the engine to be controlled precisely whilst providing high pressure injection and flexible injection timing. Important advantages are thus gained in terms of fuel economy and emissions. However, in order to operate efficiently, it is important that the pressure of fuel within the common rail is controlled accurately to a desired pressure level despite any disturbances that may be caused to the system.
In use, the relationship between the fuel pressure within the common rail (hereafter ‘rail pressure’) in response to the amount of fuel pumped into the common rail by a high pressure supply pump is a dynamic system. Typically, therefore, the high pressure fuel pump is controlled by a combination of open-loop and closed-loop control in order to fulfill the functional requirements of i) maintaining the desired rail pressure during changes of injection quantity, ii) varying the rail pressure in response to a change in pressure demand quickly and accurately, and iii) being resilient to system disturbances such as changes in fuel viscosity due to variations in temperature and fuel grade.
Although it is possible to control the pressure of fuel within the rail accurately and robustly using a combination of open-loop and control-loop control strategies, the Applicant has identified a need to provide a means to identify and characterize possible system faults and anomalous system behavior in a cost effective manner.